As a conventional vacuum degassing apparatus in a production line for glass products, the one shown in FIG. 12 has been disclosed (see, e.g., JP-A-9-142851). Specifically, this vacuum degassing apparatus 100 includes an upstream conveying pipe 130A for supplying molten glass 121, an uprising pipe 122U disposed at a downstream end portion of the upstream conveying pipe 130A for drawing up the molten glass 121 in a vertically upward direction, a vacuum degassing vessel 120 disposed to extend horizontally from an upper end of the uprising pipe 122U, a downfalling pipe 122L for guiding the molten glass 121 from a downstream end portion of the vacuum degassing vessel 120 in a vertically downward direction, a downstream conveying pipe 130B for guiding the molten glass 121 in a further downstream side from the downfalling pipe 122L, and the like.
The uprising pipe 122U, the vacuum degassing vessel 120 and the downfalling pipe 122L are covered by a casing 123, being externally insulated by a thermal insulating material. The uprising pipe 122U, the vacuum degassing vessel 120 and the downfalling pipe 122L are formed in an arched shape as a whole and pump the molten glass into the vacuum degassing vessel 120 by the principle of siphon, utilizing a differential pressure in the vacuum degassing vessel 120 to remove bubbles contained in the molten glass 121.
Metal conveying-pipes, such as the uprising pipe 122U and the downfalling pipe 122L, are made of platinum, a platinum alloy or the like to avoid reaction with the molten glass 121.
The molten glass 121, which is being supplied by the upstream conveying pipe 130A, is equalized, being stirred by a first stirrer 131a on the way to the uprising pipe. The molten glass 121 thus equalized is pumped into the vacuum degassing vessel 120 through the uprising pipe 122U to be degassed in the vacuum degassing vessel 120. The molten glass 121 thus degassed is guided to the downstream conveying pipe 130B through the downfalling pipe 122L to be conveyed to a forming process, where glass products are produced.
The upstream conveying pipe 130A, the uprising pipe 122U, the vacuum degassing vessel 120, the downfalling pipe 122L and the downstream conveying pipe 130B are metal conveying-pipes. Each of the uprising pipe 122U and the downfalling pipe 122L is fixed at upper and lower end portions thereof or in the vicinity of the upper and lower end portions thereof.
For these reasons, there is a possibility that when the molten glass 121 is conveyed at a high temperature, the uprising pipe 122U or the downfalling pipe 122L is buckled because of being subjected to compressive stress caused in an axial direction by heat.
The first one of the characteristics required for the vacuum degassing vessel 120, the uprising pipe 122U and the downfalling pipe 122L, which are in direct contact with the molten glass 121, is to avoid contamination of glass.
Additionally, equipment, which connects between adjacent regions for melting, refining or forming, to convey the molten glass to a subsequent stage, is also required to have a similar characteristic.
From this viewpoint, specific noble metal having a high melting point has been frequently employed in equipment for dealing with molten glass. In particular, when glass products, which are required to have functionality, are produced, the demand that impurities, which invade from apparatus materials, should be reduced as small as possible, is stronger.
However, the noble metal stated earlier is quite expensive. It is not acceptable to employ such noble metal in a large quantity as in iron or non-iron metal in general. For this reason, the noble metal in glass producing equipment is thinned to be employed as a lining for refractory structures or as a thin-walled cylindrical pipe, which has a complete circular cross-sectional shape that is a shape to be most difficult to be crushed.
The present invention is proposed in consideration of the problems stated earlier. It is an object of the present invention to provide a conduit for molten glass, a connecting conduit for molten glass and a vacuum degassing apparatus, which are capable of coping with expansion and contraction caused by heat, and of producing, at low cost, homogeneous and good quality glass.
With respect to the problems stated earlier, it has been disclosed that a conduit has a convex projection formed therein. For example, JP-A-2002-87826 discloses that a tube has rings formed therein to absorb thermal expansion and that the rings typically have a depth of from 2 to 3 mm. However, such a depth of from about 2 to about 3 mm is insufficient to absorb thermal expansion. The publication describes that the rings are formed by rolling. If an excessively concentrated deformed portion is caused in a ring by rolling, there is a problem that when the conduit with the deformed ring formed therein is energized for heating, electric currents are locally concentrated in a portion of the conduit with the ring formed therein to locally rise the temperature of that portion. It is believed that it is advantageous in terms of removal or separation of bubbles to provide an undulation to a portion of the inner wall of a conduit in a vacuum degassing apparatus so as to have some high degree of pitch difference. However, there is a possibility that the provision of a depth of from about 2 to about 3 mm has a problem because of poor vacuum degassing performance.
JP-A-8-67518 describes that a connecting pipe for connecting between a stirring vessel and a refining vessel or between a stirring vessel and a stirring vessel is formed with a bellows and that the connecting pipe with the bellows formed therein has both ends welded to both stirring vessels or to the stirring vessel and the refining vessel to couple the respective vessels as one unit. However, when being configured in such a coupled structure, there is a problem that it is difficult to control heating by energization since it is difficult to dispose, in a sufficient way, electrode portions for heating by energization.
This publication also describes that the bellows has a spring constant at room temperature set to be smaller than the magnitude of a deforming force per unit length caused at room temperature in the connected vessels in the axial direction of the connecting pipe. However, when the bellows is disposed in an apparatus which is not formed in a completely coupled structure, such as a vacuum degassing apparatus, the spring constant of the bellows at room temperature is not necessarily required to be smaller than the magnitude of a deforming force per unit length caused in the connected vessels in the axial direction of the connecting pipe since creep deformation or buckle is caused at the time of rising the temperature to a high level. When an attempt is made to decrease the spring constant of the bellows, it is necessary to increase the difference in diameter between a small diameter portion and a large diameter portion. This not only causes a problem in terms of cost by an increase in the usage of noble metal but also makes it difficult to secure a sufficient supply of molten glass since the path resistance is increased when the difference in diameter is excessive. Although this publication also describes that the connecting pipe has a thickness of 1 mm and a diameter of from 15 to 25 mm, it is impossible to obtain a sufficient supply of molten glass when the vacuum degassing apparatus has a pipe diameter in such a range. Although this publication also describes that the bellows is formed by, e.g., welding, there is a problem that there is a danger that when a platinum alloy, in particular a dispersion-strengthened platinum alloy, is employed in a conduit, in particular such a movable member, a crack or the like is caused by a decrease in the strength of the welded portions as known.